1. Field of the Invention
The invention relates to friend-foe interrogation for establishing radio communication links or for identifying an external system, generally a vehicle or an aircraft. The modulation of a light wave such as the laser ray is preferably used, which confers on the link a high degree of discretion and protection against jamming.
2. Description of the Prior Art
It is known to use light wave modulation for transmitting digital information or for establishing friend-foe links called IFF (identification of friend and foe).
The complete IFF link require an interrogator and a responder and is established by means of a key word. Generally, in the interrogator a laser generator is used for producing a first light wave. This wave may be modulated either in frequency or in amplitude. Frequency modulation takes place on each side of a mean value F, amplitude modulation is effected by the presence or absence of pulses. For this, a laser emitting modulator receives from a control and coding circuit a modulation signal translating the code or the key word to be transmitted. At the level of the responder, the same control and coding circuit is used for coding a monitoring key word. A laser receiver and a demodulator deliver to a correlation circuit the key word received for comparing it with the key word of the responder. Depending on this comparison, the key word is validated or not. The result of this comparison is communicated to the interrogator, for example by radio, for establishing the final communication.
Amplitude modulation has always been the most widely used, for it is easy to put into practice. But it has two major disadvantages. The first is the ambiguity of decision, resulting for example from high fluctuations of the amplitude. An insufficient pulse amplitude may be mistaken for a zero. The second disadvantage is the low information capacity of the corresponding messages. This point is particularly important in an IFF link where the duration of the message must be short. Frequency modulation consists in transmitting two or more recurrence frequencies characterizing a coding structure. But for a laser of a wave length equal to 1.06 microns, the number of recurrences is limited. The capacity of the message remains therefore small.
Furthermore, for some applications, the safety measures imposed are such that these transmission techniques and the coding means used do not provide a sufficient degree of discretion.
The aim of the invention is to provide a connection system which allows the use of a large number of different key words and in which the coding system appreciably reduces the risks of interception and decoding of the key word.
A first feature of the invention consists in using position modulation called MP, or PPM (pulse position modulation).
A second feature consists in employing a method of coding using a cyclic code and decoding by correlation of the key word without fixed reference pulse.
It is also proposed for the responder, on the one hand, to take into account parasite pulses and, on the other, not to wait for the end of reception of the key word before decoding so as to reduce the interrogation time.